The human skeleton is composed of 206 individual bones that perform a variety of important functions, including support, movement, protection, storage of minerals, and formation of blood cells. These bones can be grouped into two categories, the axial skeleton and the appendicular skeleton. The axial skeleton consists of 80 bones that make up the body's center of gravity, and the appendicular skeleton consists of 126 bones that make up the body's appendages. The axial skeleton includes the skull, vertebral column, ribs, and sternum, among others, and the appendicular skeleton includes the long bones of the upper and lower limbs, and the clavicles and other bones that attach these long bones to the axial skeleton, among others.
To ensure that the skeleton retains its ability to perform its important functions, and to reduce pain and disfigurement, fractured bones should be repaired promptly and properly. Typically, fractured bones are treated using fixation devices that reinforce the fractured bones and keep them aligned during healing. Fixation devices may take a variety of forms, including casts and external fixators for external fixation, and bone plates and/or fasteners (e.g., bone screws) for internal fixation.
Bone plates are implants that may be positioned under skin and other soft tissue for mounting on the bone adjacent the fracture. These plates may be manufactured and/or custom bent for mounting to particular regions of bone. To use a bone plate to repair a fractured bone, a surgeon (1) selects an appropriate plate, (2) reduces (sets) the fracture, and (3) fastens the plate to the bone on opposing sides of the fracture using suitable fasteners, such as bone screws, so that the bone plate spans the fracture and fragments of the bone are substantially fixed in position.
A potential disadvantage to the use of bone plates, particularly in elderly individuals, is the difficulty of fastening the plates to bone near the ends of bones. For example, due to poor bone quality (e.g., osteoporosis or osteopenia) in elderly individuals, bone plates may be fastened to bone via bone screws that are locked to plate apertures, rather than relying solely on threaded engagement with bone to hold each bone screw in position. However, bone plates with locked bone screws may create a very rigid structure that damages bone in response to bone loading, thereby loosening the bone plate.
A distinct approach to fixing fractured bones involves the use of one or more pins to span a fracture. The pins are placed into a fractured bone, with each pin spanning the fracture inside the bone. The use of pins may provide advantages over a plate-based approach to fixation. For example, pins may be more flexible than bone plates. Accordingly, pins tend to permit more micro-motion of otherwise fixed bone fragments, thereby promoting callus formation at the fracture site, which may speed healing and improve the strength of the bone after healing. In addition, pins may be less likely to damage bone than a bone plate fastened with bone screws.
However, the pin-based implants of the prior art may have certain drawbacks. For example, pins inserted into bone may not be anchored adequately to bone and thus may slip, thereby permitting too much movement of bone fragments for proper healing.